Electro-optical incremental encoders of transducers are known for measurement of linear or rotary motion or position. Such encoders include a scale of alternately light responsive and opaque segments which is cooperative with an optical head to provide an optical pattern representative of the motion of the head relative to the scale. Electrical signals derived from the optical pattern represent the relative motion, and these signals are usually processed to produce a digital count indicative of the distance the head travels relative to the scale. Scale errors, such as variations in the width and spacing of the scale segments, can cause errors in the measurement of the distance along the intended path. These errors can be caused by imprecise printing, scribing or other formation of segments on the scale, as well as by variations caused by temperature changes of the scale during operation. Such errors can become significant especially in high resolution encoders in which a high density optical pattern is provided on the encoder scale.
Scale error has been treated heretofore by adjustment in the encoder electronics to add or subtract counts as predetermined intervals of distance along the scale to alter proportionally the total count in order to minimize the error component. One method of treating scale adjustment is shown in copending U.S. patent application Ser. No. 618,509, filed on June 8, 1984 by Sidney A. Wingate, the inventor named therein, entitled: "POSITION ENCODER COMPENSATION SYSTEM," and assigned to the assignee of the present invention. In said system, depicted in FIG. 1 herein, the position encoder is automatically compensated for scale and other similar errors by altering the phase of the encoder signal by a small amount for each small increment of relative motion. The encoder provides an electrical output of four sine wave signals, +S, -S, +C and -C, which are complementary pairs of sine waves with phases displaced by 90.degree.. The signals are applied to a displacement counter 12 operative to provide an output signal for each predetermined amount of movement of the encoder head relative to the encoder scale. The displacement counter 12 provides an increment output pulse for relative motion in one direction, and a decrement output pulse for relative motion in the opposite direction. A preset input to counter 12 is provided by control 14, which provides the displacement increment for which counter 12 will provide an output. The input of control 14 is derived from a measurement of the scale error, and this input is used to determine the amount of relative movement for which a correction output is to be provided. The displacement counter 12 drives counter and switching logic which provide the requisite phase correction to the encoder signals. The output of counter 12 is applied via a reversing switch 16 to a phase control counter 18, which provides a multiple bit digital signal representative of the number of displacement increments traversed by the encoder scale relative to the head. The output of counter 18 controls the operation of quadrant switches 20 and 22 and multiple switches 28 and 30 to accomplish the desired phase correction, via resistor networks 24 and 26. The phase compensated encoder signals are then employed as the encoder output for display or other utilization purposes.